Many types of synthetic fluids useful as lubricating oils are available, including hydrogenated poly-alpha-olefins, aliphatic esters of aliphatic dibasic acids, trimethylolpropane aliphatic esters, pentaerythritol aliphatic esters, silicones, silicate esters, and phosphate esters. For a detailed discussion, see R. C. Gunderson and A. W. Hart, editors, "Synthetic Lubricants", (1962). These fluids were developed as improvements over refined mineral oils, adding such benefits as improved thermal stability, improved oxidative stability, superior volatility characteristics, better viscosity/temperature characteristics, and improved frictional properties. The use of synthetic lubricants has expanded considerably in recent years. New applications for synthetic fluids bring new demands for performance. Changes in lubricated equipment design and operating temperatures impose further requirements for upgraded fluid performance.